The present invention relates to an improved environmental control system.
In some types of environmental control system incorporating a heat pump, it may be desirable to recover heat from air from one source and to reject this heat to the space air whose environmental conditions are to be controlled. One source of air which may be suitable under certain circumstances is outside air but this can give rise to a problem which also arises in other applications where the air temperature in an evaporator may fall towards zero degrees C.degree.. In those circumstances, frosting may occur and this will drastically reduce the efficiency of the heat transfer from the air to the refrigerant in the evaporator. There have been various proposals for reducing this frosting, for example, so-called "hot gas defrost", electric heat defrost and "reversed cycle defrost". However the previously proposed means tend to involve relatively high energy consumptions.
In some environmental control systems, provision is made for space air to be re-circulated after it has been conditioned, inter alia, by the heat pump. One aspect of the present invention is concerned with providing a means whereby in such an environmental control system, advantage can be taken of the fact that space air can be recirculated to avoid the evaporator air temperature dropping sufficiently for frosting to occur. More particularly this aspect of the present invention is concerned with an environmental control system in which heat can also be recovered from an alternative source which is liable under certain circumstances to produce an evaporator air temperature which is sufficiently low for frosting to occur and provides that in such circumstances space air is re-circulated through the evaporator to maintain it at an adequate temperature, such that frost formation is inhibited.
Thus according to the first aspect of the present invention there is provided an environmental control system for controlling the condition of a space, including: a heat pump arrangement comprising an evaporator for recovering sensible and latent heat from air delivered thereto, a condensor for rejecting heat recovered by the evaporator to air to be delivered to the space; and means for delivering air from a selected source to the evaporator, the means being arranged so as, in one mode of operation, to deliver outside air to the evaporator and so that in the event of the evaporator air temperature falling to a level which could result in frosting of the evaporator, air re-circulated from the space is delivered to the evaporator to maintain the air temperature above that level.
One important area of application of the present invention is to the environmental control of swimming pool halls. In such applications, there is an important source of heat available for conditioning the space air, namely the latent heat of moisture contained in the space air. By having the space air passed through an evaporator where it is cooled sufficiently for the moisture to condense, the heat recovered can then be rejected either to the dehumidified space air or to incoming outside air or to a mixture of the two so as to provide an inflow into the swimming pool hall of heated, dryer air.
In order to maintain the environmental conditions at night, when the rate of evaporation of water from the pool is low, it is possible to use the evaporator to recover heat from the outside air, this heat then being rejected to the space air. This is, of course, the situation in such an application where the possibility of frosting of the evaporator occurs. The second aspect of this invention is concerned with the situation where in normal circumstances one would wish to recover heat from the ambient air, for example during the night mode of operation of the swimming pool environmental control system, and where the risk of evaporator frosting would occur. It seeks to use an alternative source of heat, when available, for heat recovery so that the heat recovery load on the compressor is increased. In the swimming pool application, an important source of heat is waste water from showers, wash basins etc and also the waste water which is produced during backwash, i.e. the reverse-flushing of the pool water filters which is carried out periodically. While this waste water might not be considered a major potential source of heat in ordinary circumstances, nevertheless it can provide a valuable source of heat to aleviate the heating demand on the evaporator during night operation. Thus in general terms, this aspect of the present invention provides for the recovery of heat from the waste water to enable the evaporator load to the compressor to be increased.
Thus according to a second aspect of the invention, there is provided an environmental control system for controlling the condition of a space, including a heat pump arrangement comprising an evaporator for recovering heat from air delivered thereto, a waste water store, means for recovering heat from waste water in said store, a condensor for rejecting heat recovered by the evaporator and the heat recovering means to air to be delivered to the space, the heat pump arrangement being operable in a mode in the normal course of which heat is recovered from air by the evaporator and being arranged so that when operating in that mode, at least when the air temperature is low enough for frosting of the evaporator to occur, heat is preferentially recovered from the waste water so as to increase the heat recover load on the compressor.
As will become apparent from the following description, both aspects of the present invention may be embodied in a single environmental control system used, for example, in controlling the environmental conditions of a swimming pool. The following description will be given in terms of the application of the invention to the swimming pool environment but it will be appreciated that the various techniques and features proposed could be used in a similar, analogous fashion in other applications.
Preferably the environmental control system incorporates a re-circulation duct for delivering space air to the evaporator, for delivering air from the evaporator to the condensor, and for delivering air from the condensor to the space. The means, in the first aspect of the invention, for selectively delivering air to the evaporator may then be associated with the re-circulation duct upstream of the evaporator and comprise an outside air inlet and control vanes for controlling the ratio of outside air and space air into the evaporator. During normal day-time operation, the air which passes through the evaporator is from the swimming pool hall and has the latent heat content recovered by the evaporator and this heat is then rejected to air entering the swimming pool hall by the condensor. The air heated by the condensor may be either the air from the evaporator or outside air or a mixture of the two and preferably the control circuitry of the system is arranged so that the source of air seected for heating by the condensor is the one having the higher enthalpy value. During normal night mode operation, when the rate of evaporation of water in the pool hall is very much lower, the evaporator can use the outside air is its prime source of heat to maintain a desired set point temperature in the pool hall; this night-time demand temperature may be selected somewhat less than the day-time one. According to the first aspect of the invention, when the ambient air temperature falls to a level where frosting of the evaporator should occur, a quanity of space air is allowed to re-circulate through the evaporator to maintain its temperature above the frosting threshold.
During day-time operation, if the heat recovered by the evaporator exceeds the heating demand for heating air entering the hall, the excess heat may be dumped into the pool water to raise its temperature although the system controller will define a maximum permitted water temperature to prevent a runaway situation occurring because the evaportion rate increases with the pool temperature rise and it is undesirable to let the water temperature exceed the hall air temperature. It is normal to use a removable cover on the pool when not in use to cut the rate of evaporation.
Downstream of the evaporator there may be provided an exhaust outlet for exhausting air from the evaporator to the outside, an inlet for outside air and control vanes for controlling the passage of air from the evaporator to the exhaust outlet and/or to the condensor and the passage of air from the outside air inlet to the condensor. These control vanes may be used to select the air source used to supply the evaporator. Additionally, for operation in the night mode, a direct re-circulation duct may be provided for re-circulating space air directly from the space to the condensor for heating and thence back to the space, the flow of air through the direct re-circulation duct being prevented during normal day-time operation.